Considerable microbiological research has been devoted to understanding the nutritional requirements and environmental conditions that promote selective growth of Listeria spp. Dependable selective culturing of Listeria spp. is becoming increasingly important in the food industry because of evolving federal and state regulations requiring more frequent monitoring of food-processing equipment and environments. Listeria spp. is considered to be a critical indicator of the effectiveness of industrial sanitation practices for two principle reasons: 1) organisms of the genus Listeria are ubiquitous; and 2) the species Listeria monocytogenes is pathogenic and thus a cause of concern for public health officials.
Among the bacteria of the genus Listeria spp., only the species monocytogenes is known to be pathogenic to humans. Other species of Listeria such as L. ivanovii are not generally pathogenic or are pathogenic only for animals. L. monocytogenes is a gram-positive, motile, aerobic and facultatively anaerobic bacterium which is ubiquitous in nature. It can cause various diseases in man including meningoencephalitis, low-grade septicemia, infectious mononucleosis-like syndrome, pneumonia, endocarditis, bacterial aortic aneurysm, localized abscesses, papular or pustular cutaneous lesions, conjunctivitis and urethritis.
In the past decade, L. monocytogenes has been recognized as a major food-borne pathogen. Outbreaks of listeriosis have been linked to a number of contaminated foods such as coleslaw, Mexican-style soft cheese, pasteurized millk and turkey franks. It has been isolated from fresh produce, dairy products, processed meats and seafood products. About 500 people die each year in the United States from Listerial food poisoning; the victims are usually the immunocompromised, pregnant women and neonates.
The isolation and the identification of the bacterium L. monocytogenes is a major problem in the monitoring of food hygiene and of medical bacteriology. While a number of putative media for selective culture of Listeria spp. have been described in the literature, each have disadvantages. For example, Lovett et al. describe an enrichment broth for selective isolation of Listeria spp. and U.S. Pat. No. 6,228,606 describes a method for inhibiting L. monocytogenes using a synthetic chromogenic substrate. However, these media detect every species of the genus Listeria spp. Thus, supplementary identification tests, such as microscopic, biochemical, immunological, and/or genetic tests must be used to establish the presence of the pathogenic monocytogenes species. However, these supplementary manipulations increase the length of time and cost of the analyses, require a vast number of reagents and the use of qualified personnel, and are often a source of error or at least the cause of lower precision and reliability. This is especially true when there is a very small amount of L. monocytogenes present.
Other methods for the selective culture of Listeria spp. have been described, such as Fraser and Sprerber's medium exploiting the high salt tolerance of Listeria spp., and its ability to hydrolyze esculin. Esculin is a glucoside (6-(beta-D-glucopyranosyloxy)-7-hydroxy-2H-1-benzopyran-2-one, CAS No. 531-75-9) obtained from Aesculus hippocastanum (the horsechestnut) and is characterized by its fine blue fluorescent solutions. In this approach, the beta-glucosidase activity of Listeria hydrolyzes esculin. The hydrolysis products, in combination with iron salts present in the medium, yield a black pigment that is used as a colorimetric indicator of a positive sample. Donnelly & Baigent developed a modified medium similar to the Fraser & Sprerber broth but lacking the colorimetric indicator. This medium exploits the salt tolerance of Listeria spp. in conjunction with several antibiotics to yield a medium selective for the growth of Listeria. However, these media slow the overall growth rate of Listeria cells to achieve inhibition of competitive micro-flora in the sample being tested. Further, the combination of high salt concentration and antibiotics prevents the growth of certain strains of Listeria, most notably L. ivanovii and L. grayi. 
Another complicating aspect of conventional selective media is the presence of acriflavin. Acriflavin is an acridine dye that is an effective inhibitor of competitive gram-positive bacteria such as Bacillus spp. Unfortunately, acriflavin not only is a suspected carcinogen but is also a fluorophore that is incorporated into the DNA and proteins of growing cells. Thus, acriflavin causes unwanted fluorescent interference in many fluorescence-based assays, such as enzyme-linked immunosorbent assays (ELISA) and the polymerase chain reaction (PCR). Many commercially available Listeria detection products rely upon the use of fluorescent reagents for analyte detection.
Thus, there remains a long-felt and unmet need for a Listeria-selective medium that 1) does not appreciably interfere with the growth rate of Listeria spp.; 2) does not yield bacterial biomass contaminated with interfering fluorophores; and 3) strongly inhibits the growth of non-Listeria organisms.